A radio connection can be established between a transmitter and a receiver in a radio communications system. The connection is two-directional, and one refers to a downlink that forms the connection in a direction away from a base station in the radio communications system to a mobile station, and an uplink which forms the connection in the opposite direction, i.e. from the mobile station to the base station. Transmission and reception of radio traffic for different connections takes place on radio channels that can be defined by a given frequency in an FDMA system (Frequency Division Multiple Access) or by a combination of a given frequency and a given time slot in a system that uses TDMA (Time Division Multiple Access).
The radio channels available in an FDMA system and a TDMA system can be reused. Thus, the signal strength received in a receiver on a radio channel may include a signal strength contribution from all transmitters that transmit on this radio channel. The distance between two transmitters that transmit on one and the same radio channel, the so-called repetitive distance will preferably be sufficiently great to ensure that the desired received signal is not subjected unduly to co-channel interference.
By interference is meant the sum of the signal strengths of all undesirable signals received on the radio channel used. These undesirable signals derive primarily from other connections that use the same radio channel in neighboring cells of the radio communications system.
A poor connection of unacceptable quality in a radio communications system may be due, among other things, to the fact that the ratio between the signal strength of the desired signal and the interference is too low. The signal strength ratio between the desired signal (carrier) and the disturbing signals (interference) is normally given as the C/I ratio (Carrier to Interference ratio) and is a measurement of channel quality.
The U.S. Patent Specification U.S. Pat. No. 5,157,709--Ohteru, teaches an adaptive radio communications system that includes a control station which sets up an interference matrix for the interference values between base stations. Each base station measures power levels on signals that are received on unoccupied radio channels. Information relating to received power levels on unoccupied radio channels is forwarded to the control station together with the radio channel and base station identity. The control station generates on the basis of this information an interference matrix which is used in adaptive allocation of channels to the base stations. U.S. Patent Specification U.S. Pat. No. 5,603,092 Stjernholm, also teaches a method of estimating interference. The interference is used for statistical evaluation. Measuring of traffic in other cells is carried out in both cases, i.e. in both Ohteru and Stjernholm. When frequencies are repeated by different transmitters located at different distances from a receiver, the strength of the signal received from these transmitters will vary. When several transmitters send simultaneously on one and the same channel, it is normally only possible to identify the strongest signal. The result is that only a few observations are obtained from remote cells. The method to Ohteru involves creating an interference matrix that contains information relating to mutually interfering cells and how often such interference occurs. The method does not, on the other hand, involve transmitter-receiver amplification between the cells. The method taught by Stjernholm involves measuring the magnitude of the interference. The magnitude of the interference magnitude, however, will depend on transmission power. one drawback in this regard is that dynamic power regulation results in difficulties in power level distribution, when said level changes. Furthermore, difficulties arise in measuring the relationship of a receiver to a remote transmitter when the power output of these transmitters is maintained at a low level in order to avoid interference situations.
When creating a frequency plan in a radio system for instance, it is desirable to know what affect an individual transmitter in a relatively wide area in the system will have on receivers in the system. It is also desirable to compile in a simple fashion information where relationships between each individual transmitter, irrespective of its transmitted power, and each receiver can be predicted.